The present invention relates to a method of controlling a continuously variable transmission on a motor vehicle or the like, and more particularly to a method of controlling a continuously variable transmission in combination with the control of the throttle valve of an engine.
Heretofore, continuously variable transmissions having generally been controlled such that (a) the rotational speed of an engine coupled to the continuously variable transmission will reach a target speed, (b) the rate of change of the rotational speed of the engine will reach a target rate, and (c) the speed reduction ratio or transmission ratio of the transmission will reach a target ratio.
According to another method of controlling a continuously variable transmission, the rate of change of a speed reduction ratio is controlled, the rate being calculated as the sum of a component corresponding to a predicted acceleration that is calculated from a reserved horsepower of an engine coupled to the transmission and a component corresponding to a target rate of change of the engine rotational speed (see, for example, Japanese Laid-Open Patent Publication No. 63-53343 filed by the applicant). The reserved power is a power which is available corresponding to the present accelerator opening but is not used. In other word, the engine can afford to output the reserved power under the present accelerator opening.
There is also known a method of simultaneously controlling a continuously variable transmission and the throttle valve of an engine coupled to the continuously variable transmission, so that the engine and a system for actuating the continuously variable transmission will always be controlled to operate the engine at a minimum fuel consumption rate (see Japanese Patent Publication No. 61-8305, for example).
The conventional control methods described above pose no problem insofar as the motor vehicle runs steadily or is gradually accelerated or decelerated. However, when the motor vehicle runs transiently, such as when the accelerator pedal is abruptly depressed to accelerate the motor vehicle, the rate of change of a controlled value until it reaches a target value may not necessarily best match the demand desired by the driver of the motor vehicle. Accordingly, under transient running conditions of the motor vehicle, the driver may not experience the desired feeling as to the driving conditions of the motor vehicle. For example, according to the conventional control methods, when the accelerator pedal is depressed, the rotational speed of the engine is increased to a speed value corresponding to the depth to which the accelerator pedal is depressed (the amount of depression of the accelerator pedal), and then the motor vehicle is accelerated while the engine rotational speed is being kept constant. Therefore, the rotational speed of the engine or the rate of change of the rotational speed of the engine does not match the motor vehicle acceleration as felt by the driver, and the depression of the accelerator pedal also does not match the motor vehicle acceleration as felt by the driver.
In view of the fact that the accelerator pedal depression is an indication of the intention of the driver for acceleration or deceleration, there has been disclosed a transmission control method in which a target acceleration is established depending on the depth to which the accelerator pedal is depressed, and the transmission is controlled to arrive at the target acceleration (see Japanese Patent Publication No. 62-52177).
According to this transmission control method, however, inasmuch as a target acceleration which is once established remains at one value, when the accelerator pedal is depressed and thereafter held in the depressed position, the target acceleration remains unchanged even if the speed of the motor vehicle goes higher as the vehicle is accelerated. As a result, the target acceleration deviates from a motor vehicle acceleration which is desired by the driver. Furthermore, when the accelerator pedal is released back for deceleration, the transmission is controlled for a lower speed reduction ratio, i.e., for a top position, and hence the motor vehicle is decelerated without much of an engine brake being applied.